1. Field of the Invention
The present invention relates to mullite fibers. More particularly, the present invention relates to apparatus for and a method of producing mullite fibers by inviscid melt spinning.
2. General Background of the Invention
The aluminosilicate (SiO.sub.2 --Al.sub.2 O.sub.3) system is one of the most widely studied binary metal oxide systems, due to the many structural and refractory ceramics that contain a majority of these two Compounds. Of the aluminosilicates, mullite (Al.sub.2 [Al.sub.2+2x Si.sub.2-2x ]O.sub.10-x where 0.25&lt;x&lt;0.4, representing, 69-85% Al.sub.2 O.sub.3 by weight, hereinafter referred to as "mullite") possesses excellent high temperature mechanical properties such as creep resistance. The potential advantages of mullite as a high temperature structural material have not yet been fully realized, however, due in part to the inherent difficulty in processing mullite into a fiber form for Uses Such as reinforcement in composites.
There are two existing processes for the manufacturing, of fibers in the SiO.sub.2 --Al.sub.2 O.sub.3, system: melt-spinning processes and slurry or solution processes. Melt-spinning methods are usually used for fibers containing 45-60 wt % Al.sub.2 O.sub.3. Higher alumina content fibers cannot be formed by these methods because of the higher melting point and lower viscosity. Two other methods developed for high-Al.sub.2 O.sub.3 fibers have advanced to commercial production, namely, the slurry and solution- or sol-gel-spinning processes. The slurry processes involve extruding aluminosilicate slurries, then drying and heating the resulting fibers to produce a polycrystalline filament. The solution or sol-gel processes involve extruding and drawing into fibers a viscous solution of aluminum compounds, followed by drying and heating at high temperature. However, multiple process steps make slurry and solution processes slow, uneconomical and difficult to control. Additionally, the difficulty in controlling the fiber geometry and impurities and heterogeneous microstructures in the final products leading to their poor mechanical properties are also drawbacks of the slurry and solution methods. Other methods, such as the Edge-defined Film Growth (EFG) technique and the Unidirectional Freezing of a Hydrogel (UFH) technique, might also be used for the high-Al.sub.2 O.sub.3 fiber production, but their low process productivity and expensive equipment costs make them difficult to commercialize.
Inviscid Melt Spinning (IMS) has been used to produce fibers of a wide variety of materials ranging from metals such as steel, to ceramics such as barium titanate. The IMS process utilizes a reactive gas environment to stabilize low viscosity melts, typically metals, alloys or metal oxides, against Rayleigh breakup. The type of reactant gas varies, depending on the melt chemistry, as do the processing parameters (melt temperature, extrusion pressure, crucible material), and the process must be carefully modified for each type of fiber chemistry.
This invention includes among its objectives apparatus for producing mullite fibers utilizing a reactive gas stabilization process.
Another objective of this invention involves a crucible design which eliminates volatilization of one of the key melt components, specifically SiO.sub.2, at processing temperatures, thereby maintaining appropriate melt composition during the fiber forming process.
The following U.S. Patents are incorporated herein by reference: U.S. Patent Nos.
4,101,615 3,626,041 5,055,348 5,582,912 3,668,096 3,593,775 4,104,355 3,727,292;
and
Inviscid Melt Spinning Patents:
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